1. Field of the Invention
The present invention relates to a document reading apparatus, and particularly relates to a structure for suppressing radiated electrical (electromagnetic) noise.
2. Description of the Related Art
Conventionally, an image reading apparatus reads a bound original by scanning the bound original while moving a reading unit constituted by a carriage member supporting an image sensor. This has been proposed in, for example, Japanese Patent Laid-Open No. 8-195860. This image sensor includes a light source that linearly illuminates an original document (hereinafter referred to as an original) in a main-scanning direction; an imaging means that images light reflected from the original illuminated by the light source; a photoelectric conversion means that converts image information about the original into a read image signal by making the reflected light imaged by the imaging means enter the photoelectric conversion means; and an image signal sending means that sends the image signal created by the photoelectric conversion. In particular, the reading unit is connected to an image signal receiving unit via a flexible cable serving as the image signal sending means and being flexible. An image reading apparatus having such a structure has an advantage that the thickness of the apparatus can be reduced.
However, assuming such a structure, the reading unit is put in an electrically floating state in a high-frequency region, and so there is a possibility that a driving signal for the image sensor may cause radiated electrical noise. A driving signal for driving the light source or the image sensor, which are used to read an image, requires a large current because of the photoelectric conversion that is performed. Moreover, in order to realize high-speed reading, a high-frequency signal is necessary. Therefore, ICs for driving the light source means or the image sensor repeatedly send/receive high-frequency and large-current signals therebetween each time an image is read, and thus there is a remarkable amount of radiated noise.
Japanese Patent Laid-Open No. 2005-17572 proposes a measure to reduce radiated noise in an image reading apparatus. Specifically, according to Japanese Patent Laid-Open No. 2005-17572, a flexible cable is kept in contact with a housing and the like while being moved. It is stated that the occurrence of radiated noise is thus reduced.
However, radiated noise does not occur only from the flexible cable. The reading unit moves in a sub-scanning direction during an image reading operation, and the magnitude of the radiated noise changes depending on the sub-scanning position of the reading unit. That is to say, the radiated noise may increase substantially depending on the sub-scanning position. The inventor of the present invention conducted research and found that this is due to electromagnetic coupling between the reading unit and the housing that has conductivity.